Water drops on Lupine

Wild lupine leaves after a summer rain

Lupines have a waxy cuticle on the surface of their leaves.  Epicuticular wax is common in many plants, which is why rain beads the way it does on their surfaces.  The drops, which are circular, appear so because of the high cohesive properties of water - the tendency for water to stick to itself.  This is why you get a stream of water when you turn on a tap, why water falls from the sky as drops, and why water has such a strong surface tension.

The waxy surface of plants exists for a number of reasons.  It prevents water from leaving the plant through evaporation.  Water certainly leaves plants regularly, but through an active process called transpiration.  The cuticle also protects the plant against insect attacks, pathogens, and environmental factors like UV radiation.  Wax is hydrophobic, meaning that it repels water.  When you wax a car, water beads up on its surface.  The wax helps prevent substances from sticking to the car's exterior.

So, does the waxy cuticle prevent water from either entering or exiting plants all together?  The anwser is, no.  Plant leaves have a remarkable surface structure involving stomata - a bracketing pair of cells that regulates gas exchange with the air.  Carbon dioxide goes in during the day to facilitate photosynthesis and leaves at night during respiration.  The same is true for oxygen, although in reverse, as oxygen is a by product of photosynthesis and needed for respiration.  Water vapour is also released from the pores.

The movement of water through a plant is critical for the plant to get nutrients from the roots.  Those nutrients are not moved independently, but rather are dissolved in water and are transported up through the xylem.  Conversely, water is used to send sugars downward to the roots through another group of transport cells called the phloem.  Since more water comes up through the xylem than what goes down through the phloem, that extra water ends up in one of two places.  It either is used in photosynthesis or is released through transpiration.

Although plants can absorb water through the leaves, it is an inefficient process for most species because of that waxy layer.  The nice thing about it, though, is that it does make surface water look rather pretty when it rains outside.

Thanks for reading.

Eric Svendsen     www.ericspix.com